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Advanced wireless communication systems for commercial applications such as WCDMA (Wideband Code Division Multiple Access) and LTE (Long Term Evolution) cellular have been designed to maximize the data capacity, range and immunity to interference for a given radio channel. Hence the transceivers used in the terminals for these systems have to deliver excellent performance while meeting the cost targets required for successful commercial adoption. The RF (Radio Frequency) receiver section has to deliver very low noise to allow a weak desired signal to be received while simultaneously having high linearity (power handling capability) and high rejection of unwanted signals to avoid distortion when the interferers are present as well as the desired signal. Contributing to the relatively high terminal cost is the low integration of current cellular radios. WCDMA and LTE are FDD (Frequency Division Duplex) systems meaning that both the transmitter and receiver are operating at the same time. This presents a particular problem in that the high powered transmit signal has the potential to distort the weak desired signal being received through the limited receiver linearity. The second point is that noise on the high powered transmit signal will appear in the desired receiver band. Both of these effects corrupt and can potentially make the desired signal unrecoverable.
A standalone electronic device known as a duplexer is used to isolate the receiver from the high power transmit signal but the isolation between transmitter and receiver that is provided by low area, low cost duplexers is limited, with a minimum isolation at the TX frequency of 50 dB. Taking the example of a Class-3 WCDMA terminal, the power delivered to the antenna in transmit mode is +24 dBm. Allowing for 2 dB insertion loss resulting from the duplexer and the switch, the power at the PA (Power Amplifier) output has to be +26 dBm. This means that the receiver will see a TX leakage at its input of up to −24 dBm while simultaneously receiving a desired signal at −98 dBm.
Today's WCDMA and LTE transceivers deal with the transmitter blocking problem by using an external transmit SAW filter to eliminate transmit noise in the desired receive band and an external LNA (Low Noise Amplifier) and receive SAW (Surface Acoustic Wave) filter to achieve sufficient receiver linearity. This means a tri-band transceiver with 2 port receiver diversity requires 9 SAW filters and 6 LNAs which is obviously contrary to the requirement for a highly integrated, low cost cellular terminal. Such a terminal requires a silicon integrated radio that has the ability to deal with the TX blocker and TX noise in the receive band without the need for external SAW filters, the so-called “SAW-less” radio.
The need to deliver commercial radios that are low cost and suitable for integration within a larger single IC known as an SoC (System-on-a-chip) in which radio, digital and other functions are combined means implementation in a deep sub-um CMOS process. The most advanced CMOS process node currently being used for near term commercial radio development is termed 40 nm, this refers to the minimum feature size on the mask that can be processed. The 40 nm node provides the opportunity of low cost digital logic and memory integration as well as high frequency core transistors but it also brings with it a low supply voltage limit and high parasitic routing resistance. Both of these features are significant limitations when it comes to delivering a low noise, high linearity radio.
SoCs for wireless applications designed in 40 nm technology allow for the cost effective integration of various applications e.g. cellular, GPS, Bluetooth, WiFi, FM etc. given the small digital logic and memory area resulting from scaling. However, the need to include a radio for each of these applications, integrated on the one silicon die brings up the problem of co-existence between the separate radios and between each radio and the digital baseband which in itself is an efficient generator of interference. A technique is required that will allow the separate radio receivers to operate successfully in this hostile environment i.e. to receive the desired air-borne signal with the required signal to noise and interference ratio while on-chip sources of interference are present e.g. 2 GHz digital logic clock, 2.48 GHz WiFi transmission.
As regards interference sources that have the potential to degrade radio receiver performance, these are not limited to those that are self-generated e.g. cellular Tx blocker or 2 GHz digital logic clock. Some of the most troublesome interferers are air-borne and these can be picked up by the antenna and delivered to the receiver in the same way as for a desired signal. GPS is an example of an application that is extremely sensitive to disruption from external interferers, legally and illegally generated. For example, the received satellite signal strength for GPS is typically −130 dBm. Commercial GPS receivers include a certain degree of processing gain as a result of the signal spreading technique used that gives the receiver some immunity to interference. The level of immunity varies between receiver types but is typically around 40 dB meaning that an external interferer in-band or close to band edge can prevent operation at a very modest power level of −90 dBm. A technique is required that will provide a high degree of on-chip (hence low cost) receiver selectivity at the RF frequency for the particular application, in this particular example 1.575 GHz for GPS. This scenario also applies to other radio receivers e.g. LTE.
In reality, any of the receiver functions may have to deal with more than one blocking signal simultaneously, for example the cellular receiver in FDD mode will need to tolerate its corresponding Tx blocker and possibly the 2 GHz digital logic clock.